Medical sensor and technique for using the same

ABSTRACT

A sensor is provided that is appropriate for transcutaneous detection of tissue or blood constituents. A sensor for tissue constituent detection may include a gas collection chamber with a conduit to a sensing component and a conduit from the sensing component to the chamber. A sensor as provided may also include a barrier layer to prevent water from infiltrating the sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/735,621, filed Nov. 10, 2005, the disclosure of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to medical devices and, moreparticularly, to sensors used for sensing physiological parameters of apatient.

2. Description of the Related Art

This section is intended to introduce the reader to various aspects ofart that may be related to various aspects of the present invention,which are described and/or claimed below. This discussion is believed tobe helpful in providing the reader with background information tofacilitate a better understanding of the various aspects of the presentinvention. Accordingly, it should be understood that these statementsare to be read in this light, and not as admissions of prior art.

In the field of medicine, doctors often desire to monitor certainphysiological characteristics of their patients. Accordingly, a widevariety of devices have been developed for monitoring many suchcharacteristics of a patient. Such devices provide doctors and otherhealthcare personnel with the information they need to provide the bestpossible healthcare for their patients. As a result, such monitoringdevices have become an indispensable part of modern medicine.

Physiological characteristics that physicians may desire to monitorinclude constituents of the blood and tissue, such as oxygen and carbondioxide. For example, abnormal levels of carbon dioxide in the blood maybe related to perfusion problems. Thus, assessment of carbon dioxidelevels may be useful for diagnosing a variety of clinical states relatedto the circulation. Carbon dioxide and other blood constituents may bedirectly measured by taking a blood sample, or may be indirectlymeasured by assessing the concentration of those constituents in thetissue or respiratory gases. For example, carbon dioxide in thebloodstream equilibrates rapidly with carbon dioxide in the lungs, andthe partial pressure of the carbon dioxide in the lungs approaches theamount in the blood during each breath. Accordingly, physicians oftenmonitor respiratory gases during breathing in order to estimate thecarbon dioxide levels in the blood.

However, estimation of carbon dioxide by respiratory gas analysis hascertain disadvantages. It is often inconvenient to measure carbondioxide in samples collected from an intubation tube or cannula.Although these methods are considered to be noninvasive, as the surfaceof the skin is not breached, the insertion of such devices may causediscomfort for the patient. Further, the insertion and operation of suchdevices also involves the assistance of skilled medical personnel.

Carbon dioxide in the blood that diffuses into the tissue may also bemeasured transcutaneously by sensors placed against a patient's skin.While these sensors are easier to use than respiratory gas sensors, theyalso have certain disadvantages. For example, these sensors may besensitive to the infiltration of water or bodily fluids, particularlywhen applied to a mucosal surface.

SUMMARY

Certain aspects commensurate in scope with the originally claimedinvention are set forth below. It should be understood that theseaspects are presented merely to provide the reader with a brief summaryof certain forms of the invention might take and that these aspects arenot intended to limit the scope of the invention. Indeed, the inventionmay encompass a variety of aspects that may not be set forth below.

There is provided a system that includes: at least one gas collectionchamber into which a tissue constituent is able to diffuse, wherein thegas collection chamber is adapted to be placed proximate to a tissue; anefferent conduit adapted to transfer the tissue constituent from the gascollection chamber to at least one sensing component, wherein thesensing component is adapted to provide a signal related to the tissueconstituent; an afferent conduit adapted to transfer the tissueconstituent from the sensing component to the gas collection chamber;and a motive force structure adapted circulate the tissue constituentthrough the system, wherein the motive force structure is adapted to beoperatively connected to at least one of the efferent conduit, theafferent conduit, or the sensing component.

There is also provided a monitoring device that includes: a monitor; anda system adapted to be coupled to the monitor, the system including: atleast one gas collection structure adapted to be placed proximate to atissue; and an efferent conduit adapted to transfer gas from the gascollection structure to a sensing component, wherein the sensingcomponent is adapted to provide a signal related to a tissueconstituent; and an afferent conduit adapted to transfer gas from thesensing component to the gas collection structure.

There is also provided a method that includes: transferring a tissueconstituent in a gas collection chamber to at least one sensingcomponent not located in the gas collection chamber, wherein the sensingcomponent is adapted to provide a signal related to the tissueconstituent.

There is also provided a sensing system component that includes: atleast one gas collection chamber into which a tissue constituent is ableto diffuse, wherein the gas collection chamber is adapted to be placedproximate to a tissue; a first conduit in communication with the gascollection chamber comprising a connector located distally from the gascollection; and a second conduit in communication with the gascollection chamber comprising a connector located distally from the gascollection chamber.

There is also provided a method of manufacturing a sensing systemcomponent that includes: providing at least one gas collection chamberinto which a tissue constituent is able to diffuse, wherein the gascollection chamber is adapted to be placed proximate to a tissue;providing a first conduit in communication with the gas collectionchamber comprising a connector located distally from the gas collection;and providing a second conduit in communication with the gas collectionchamber comprising a connector located distally from the gas collectionchamber.

There is also provided a sensor that includes: a sensor body comprisingat least one gas collection chamber adapted to be placed proximate to atissue; a sensing component disposed on the sensor body adapted toprovide a signal related to a tissue constituent; and a barrier layerdefining at least part of a surface of the gas collection chamber,wherein the barrier layer is substantially impermeable to water.

There is also provided a system that includes: a monitor; and a sensoradapted to be coupled to the monitor, the sensor including: a sensorbody comprising a gas collection chamber adapted to be placed proximateto a tissue; a sensing component disposed on the sensor body adapted toprovide a signal to the monitor related to a tissue constituent; and abarrier layer defining at least part of a surface of the gas collectionchamber, wherein the barrier layer is substantially impermeable towater.

There is also provided a method of measuring a tissue constituent thatincludes: diffusing a tissue constituent through a barrier layer that issubstantially impermeable to water, wherein the barrier layer defines atleast part of a surface of a gas collection chamber; and providing asignal related to the tissue constituent with a sensing element disposedon the gas collection chamber.

There is also provided a method of manufacturing a sensor that includes:providing a sensor body comprising a gas collection chamber adapted tobe placed proximate to a tissue; providing a sensing component disposedon the sensor body adapted to provide a signal related to a tissueconstituent; and providing a barrier layer defining at least part of asurface of the gas collection chamber, wherein the barrier layer issubstantially impermeable to water.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading thefollowing detailed description and upon reference to the drawings inwhich:

FIG. 1 illustrates a perspective view of a patient using a sensor fordetection of a physiological tissue constituent according to the presentinvention;

FIG. 2 is a schematic cross-sectional view of the sensor of FIG. 1;

FIG. 3A illustrates a schematic view of an embodiment of a sensoraccording to the present techniques;

FIG. 3B illustrates a view of an exemplary disposable portion of thesensor of FIG. 3A;

FIG. 4 is a flow chart of a method of operating a sensor according tothe present invention;

FIG. 5A-5B illustrate an alternate configuration of a tissue constituentcollection portion of a sensor according to the present techniques;

FIG. 7 illustrates a perspective view of a patient using a sensorincluding a barrier layer for detection of a physiological tissueconstituent according to the present invention;

FIG. 8 is a schematic cross-sectional view of the sensor of FIG. 7;

FIG. 9 is a sensor including multiple collection chambers for tissueconstituent detection; and

FIG. 10 illustrates a physiological constituent detection system coupledto a multi-parameter patient monitor and a sensor according toembodiments of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It, should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

A sensor and/or sensing system is provided herein that may assess atissue constituent content with a sensing component that is adapted toprovide a signal related to the tissue constituent. In certainembodiments of the invention, the sensing system may include acollection chamber placed against the tissue and a sensing componentthat is connected to the chamber by a conduit with a return conduitbetween the sensing component and the collection chamber. The collectionchamber is able to capture a volume of volatile tissue constituents asthey diffuse out of the tissue. When the concentration of the tissueconstituents in the gas collection chamber and throughout the sensingsystem is substantially equal to the concentration of those constituentsin the tissue, the sensing system is equilibrated

Such a system may provide multiple advantages. By separating the sensingcomponent from the tissue constituent collection chamber, a sensor maybe more versatile. For example, a sensing component may be easilyexchanged for an alternate sensing component without disrupting thecollection of the tissue constituent. This may be advantageous when asensing component needs to be maintained or serviced, and the healthcareprovider does not wish to disrupt physiological monitoring whilereplacing the sensing component. Additionally, the separation of thesensing component from the tissue constituent collection chamber mayhelp reduce water infiltration into the sensing component.

Sensors according to the present techniques may transcutaneously sensetissue gases or other tissue constituents in a tissue layer and providean electrical and/or visual signal. For example, carbon dioxide andother constituents in the bloodstream may diffuse through the tissue andmay dissolve into any liquids that may be found at the surface of thetissue. Thus, the levels of carbon dioxide in the tissue may serve as asurrogate marker for carbon dioxide levels in the bloodstream. A sensoraccording to the present techniques placed proximate to a tissue surfacemay capture and measure carbon dioxide that would otherwise diffuse intothe airstream or other surrounding airspace.

Generally, it is envisioned that sensors according to the presenttechniques are appropriate for use in determining the presence or levelsof tissue constituents in a variety of tissues. The sensor may be placedagainst the tissue, either manually, mechanically, adhesively, orotherwise, forming a seal to prevent the carbon dioxide from diffusingaway. For example, a sensor may be used in the upper respiratory tractor the gastrointestinal tissue, including the oral and nasal passages.These passages may include the tongue, the floor of the mouth, the roofof the mouth, the soft palate, the cheeks, the gums, the lips, theesophagus and any other respiratory or gastrointestinal tissue. Further,a sensor as described herein is appropriate for use adjacent to orproximate to any mucosal surface, i.e., patient surfaces that include amucous membrane or surfaces that are associated with mucus production.In addition to the respiratory tract, mucosal surfaces may includevaginal, rectal, or gastrointestinal surfaces.

Sensors as provided by the present techniques may be disposable,reusable, or partially disposable. In addition, the sensors may beappropriate for short-term or for longer-term monitoring. When used forlong-term monitoring, the sensor may be applied to the patient's tissueeither by mechanical clamping or by a suitable adhesive, such as amucoadhesive, or by any other suitable holding device, such as a clip.

In additional to carbon dioxide monitoring, sensors and sensing systemsas provided herein may be used to monitor oxygen, carbon monoxide,ethanol, or anesthetic gases (such as isoflurane, halothane, desflurane,sevoflurane and enflurane) that may diffuse transcutaneously.Additionally, these sensors and/or sensing systems may be used tomonitor volatile products of metabolism (such as ketones, alcohols,lactones, terpenes, furans, dimethyl sulfone, pyrrole, and allylisothiocyanate), as well as volatile xenobiotics and their metabolites.Further, these sensors may be useful in monitoring the levels ofparenterally administered or enterally administered therapeutic agents.

For example, FIG. 1 illustrates the placement of a gas collectionchamber 12 of a sensor 10 on a buccal surface in order to assess atissue gas, for example carbon dioxide, in the tissue, blood orinterstitial fluid. Specifically, FIG. 1 shows an embodiment of a sensor10 including a gas collection chamber 12 and a conduit 14 incommunication with a sensing component 16. The conduit 14 may be adaptedto transport gases from the gas collection chamber 12 to a distalsensing component 16. The collected gases may diffuse through theefferent conduit 14 a that is connected to the collection chamber, andthe gases may then be further assessed and/or measured by the sensingcomponent 16, discussed in more detail below. The collected gases maythen circulate back to the gas collection chamber 12 through theafferent conduit 14 b. The gas collection chamber 12 may be suitablysized and shaped such that a patient may easily close his or her moutharound the sensor with minimal discomfort.

The gas collection chamber 12 is secured to the mucosal tissue 18 suchthat the area covered by the gas collection chamber 12 creates a seal 13to prevent environmental air flow out or into of the gas collectionchamber 12, thus preventing tissue gases at the gas collection chamber12 placement site from dissipating into the airstream or being diluted,which may lead to inaccurate measurements. Further, the gas collectionchamber's 12 tissue seal may also prevent respiratory gases or oralfluids from entering the sensor 10A.

Tissue constituents 22 may be transferred through a conduit 14, whichmay include tubes or tube segments. The conduit 14 may include, forexample, medical grade catheter tubing, polyethylene, polypropylene orvinyl. The efferent conduit 14 a and the afferent conduit 14 b may bedisposed on any appropriate location on the gas collection chamber 12.For example, the efferent conduit 14 a and the afferent conduit 14 b maybe parallel or perpendicular to each other. Generally, the conduit 14may be relatively impermeable to the tissue constituent. This may beaccomplished by selecting a conduit 14 made from an appropriate materialor by applying a sealing coat to the conduit 14. The conduit 14 mayinclude gas-impermeable plastics such as PET. Appropriategas-impermeable coatings may include Funcosil® (available from Remmers,Loeningen, Germany). Such coatings may be applied to the conduit 14 inany appropriate manner.

A cross-sectional view of the sensor 10A is shown in FIG. 2. The housing20 is formed to provide a surface that is suitably shaped to be securedagainst a mucosal tissue 18. The housing 20 may be any suitable materialthat is generally suited to the aqueous environment of the mucousmembrane. For example, the housing 20 may be formed from polypropylene,polyethylene, polysulfone or similar polymers. Generally, the housing 20should be substantially impermeable to tissue constituents, shown byarrows 22, such that the sensor 10A may collect tissue constituents 22,such as tissue gases, for a sufficient period of time to allow fordetection and measurement. Hence, it may be advantageous to coat thesensor 10A with additional sealants to prevent leakage of the tissueconstituents 22. The housing 20, once secured to the tissue, forms acollection chamber 12 that traps tissue constituents 22 that diffusethrough the mucosal tissue 18. The trapped tissue gas 22 may then betransferred to the sensing component 16, which is coupled the gascollection chamber 12 by the efferent conduit 14 a.

In the depicted embodiment, the sensing component 16 is not located onor within the gas collection chamber 12. When the housing 20 iscontacted with a tissue sensor site, blood or tissue constituents 22perfuse through the tissue and enter the collection chamber 12. Thesensing component 16 is adapted to respond to the presence of the bloodor tissue constituents 22 collected in the gas collection chamber 12 andto provide a signal, as discussed in more detail below. The sensingcomponent 16 is sensitive to the presence of a tissue constituent 22 andmay be capable of being calibrated to give a response signalcorresponding to a given predetermined concentration of the tissueconstituent. In certain embodiments, the signal may be related to theconcentration or level of the tissue constituent 22, or the partialpressure of the tissue constituent 22.

In certain embodiments, the gas collection chamber 12 may includematerials that function as a barrier layer 28 that are hydrophobic orotherwise water-resistant, but that are permeable to carbon dioxide. Forexample, a barrier layer 28 may form a contact surface of the sensor 10Athat prevents water from entering the sensor 10A. In such an embodiment,carbon dioxide in the tissue can perfuse through the contact surface toenter the gas collection chamber 12. In one embodiment, it is envisionedthat the ratio of water permeability to carbon dioxide permeability of abarrier layer 28 may be less than 1:1, and in certain embodiments, theratio may be less than 1:10. Suitable materials include polymers, suchas polytetrafluorethylene (PTFE). Other suitable materials includemicroporous polymer films, such as those available from the LandecCorporation (Menlo Park, Calif.). Such microporous polymer films areformed from a polymer film base with a customizable crystallinepolymeric coating that may be customized to be highly permeable tocarbon dioxide and relatively impermeable to water. In one embodiment,the barrier layer 28 may be a relatively thin PTFE material such asplumber's tape (0.04 mm). In other embodiments, the barrier layer 28 maybe a PTFE material such as Gore-Tex® (W. L. Gore & Associates, Inc.,Newark, Del.) or plumber's tape. Alternatively, the barrier layer 28 maybe formed from a combination of appropriate materials, such as materialsthat are heat-sealed or laminated to one another. For example, thebarrier layer 28 may include a PTFE layer with a pore size of 3 micronsand a second PTFE layer with a pore size of 0.1 microns. Additionally,in certain embodiments, a sensor 10A may also include a porous substrate29 that is permeable to a wide variety of tissue constituents 22. As abarrier layer 28 may be quite thin, the porous substrate may beadvantageous in providing rigidity and support to the barrier layer 28film. The porous substrate may be adhered, laminated, or otherwiseattached to the barrier layer 28. In certain embodiments, the poroussubstrate may be disposed on the tissue-contacting side of the barrierlayer 28. Suitable materials for the porous substrate include paper,plastics, or woven materials.

In certain embodiments, the barrier layer 28 or porous substrate 29 mayinclude a tissue irritant or other agent or structure that increasesblood flow to the tissue at the gas collection chamber 12 placementsite. The agent may include a counterirritant, such as a mixture ofmethyl salicilate and menthol (12% methyl salicilate, 9% menthol) in acream base is applied to the patient's skin at the chosen sensor site. Acream of this type is sold in retail drug stores under the trademark ICYHOT. Other contemplated agents of this type may include heaters, such asmechanical or chemical heaters, that increase blood perfusion inresponse to lowered tissue temperatures.

The sensing component 16 may be disposed on or within any appropriatesubstrate that provides a suitable contact area with which the tissueconstituent 22 may interact, react, or otherwise come into the proximityof the sensing component 16. For example, in embodiments in which thesensing component includes a chemical indicator, it may be appropriateto include, as part of a holder for the sensing component 16, atransparent viewing window for the healthcare provider to view a changein color of the indicator. In embodiments in which the sensing componentincludes an optical detection system, it may be appropriate to disposethe sensing component in a chamber.

Although the tissue constituent 22 may diffuse and circulate through theconduit 14 to the sensing component 16 without a drawing force, such aprocess may be lengthy. Thus, it may be advantageous to provide a motivedevice, such as a pump, within the sensor. FIG. 3 is a schematic diagramof the exemplary sensor 10A that provides a pumping and sensingcomponent system 33 to draw the tissue constituent 22 to the sensingcomponent 16. A flow regulator 32, which may be a valve or any othersuitable device, and pump 34 are connected into or between segments ofconduit 14 to maintain a desired flow velocity of the stream of tissueconstituent 22 to the sensing component 16. As shown, the flow regulator32 is connected to a pump 34. The pump 34 is in turn interposed betweensections of the conduit 14, which is connected to the sensing component16.

The ends of the conduit 14 segments may be secured to connectors 30, asshown in FIG. 3, which may be clamped to prevent leaking of the tissueconstituent 22 to the outside. Connectors 30 a and 30 b are exemplary,and it should be understood that the connectors 30 and conduit 14 may bearranged along the sensor in any manner that is convenient for the user.For example, it may be advantageous to provide additional connectors 30in order to allow a healthcare provider to easily swap out to service,clean, or exchange any part of the sensor 10A, including the flowregulator 32, pump, 34, or sensing component 16.

In certain embodiments, it may be advantageous to exchange a firstsensing component 16 adapted to sense carbon dioxide for a secondsensing component 16. The second sensing component 16 may also sensecarbon dioxide, but may operate by a different sensing mechanism.Alternatively, the second sensing component 16 may be adapted to sense adifferent tissue constituent, such as carbon monoxide or oxygen.Further, in an alternate embodiment (not shown), the sensor 10A may havemultiple sensing components 16 in series.

In certain embodiments, the pump 34 and flow regulator 32 may beadjusted so that the flow is maintained at the desired rate. Onesuitable flow regulator is orifice/needle valve model F-2822-41-B80-55available from Air Logic, Racine, Wis., which can be adjusted to obtaina desired gas flow rate in the range of up to 40-60 m/min. One suitablepump is model NMP 05 diaphragm micro pump, available from KNF Neuberger,Inc, Princeton, N.J., which has a free flow capacity of 0.4 L/min. Thepump 34 and flow regulator 32 may be located anywhere in the flow streamof the sensor 10A. Generally, the pumping system is substantially sealedto prevent leaking of the tissue constituent 22 to the outside ordilution by entraining of fresh gas. In other embodiments, any suitablemotive force structure may be appropriate for use with the presenttechniques. For example, suitable motive force structures includegravity pumps, one-way valves, kinetic motion pumps, or piezoelectricpumps.

In certain embodiments, the pump 34, flow regulator 32, and sensingcomponent 16 may be connected to a processor 40. The processor may bepart of a monitor or multi-parameter monitor, as discussed in detailbelow. The processor 40 may receive signals related to the outputsignals from sensing component 16, corresponding to the tissueconstituent 22 concentration or partial pressure concentration.Additionally, the processor 40 may control the flow of the vacuum andsensing component system 33. It should be understood that the processor40 may be adapted to determine a suitable equilibration time of thesensor 10A by comparing the equilibration time to stored equilibrationcurves that may be empirically obtained. Additionally, in certainembodiments, the concentration of the tissue constituent 22 may beextrapolated from a concentration curve obtained by the sensor 10Aduring the pre-equilibration period, as such a curve will start toplateau as it approaches the equilibrated state.

In certain embodiments, the gas collection chamber 12, the conduit 14,or the connectors 30 may include a calibration element 42, such as acoded resistor or EEPROM or other coding devices (such as a capacitor,inductor, PROM, RFID, a barcode, parallel resonant circuits, or acolorimetric indicator) that may provide a signal to the processor 40related to the volume and other characteristics of the gas collectionchamber 12 that may allow the processor 40 to determine the appropriatecalibration characteristics for the sensor 10A. Generally, such acalibration element 42 may be located on a disposable portion of thesensor 10A, shown in FIG. 3B, that may include the gas collectionchamber 12, the conduit segments 14 attached to the gas collectionchamber 12, or any connectors 30 proximate to the gas collection chamber12. In such an embodiment, for example when the calibration element isdisposed on the connector 30 as shown, the connector 30, conduit segment14, and the calibration element 42 may be constructed as a unitaryassembly such that the calibration element 42 may be inseparable fromthe gas collection chamber 12. Further, the calibration element 42 mayinclude encryption coding that prevents a disposable part of the sensor10A from being recognized by a processor 40 that is not able to decodethe encryption. Such encryption coding is described in U.S. Pat. No.6,708,049, which is hereby incorporated by reference in its entirety.

The sensing component 16 may also include a calibration element (notshown) that provides information to the processor 40 that may includethe type of tissue constituent 22 that is being analyzed or othercharacteristics of the sensing component 16. For example, such a sensingcomponent 16 calibration element may send a signal to the processor 40to employ a certain correction algorithm for calculating theconcentration of the tissue constituent 22. Such a correction algorithmmay be appropriate when the sensing component 16 includes a chemicalindicator that consumes a percentage of the tissue constituent 22 whileactively measuring it. As the consumption of the tissue constituent 22by the sensing component 16 may alter the equilibration state of thesensor 10A, the correction algorithm may mitigate such effects on thesensing component 16 output signal. In an alternative embodiment, acorrection algorithm may also be employed if a sensing component 16generated the tissue constituent 22 during measurement. Further, acorrection algorithm may account for any minimal leakage of tissueconstituent 22 in the system.

The collection chamber 12 is part of a substantially closed environment,as the conduit 14, and the sensing component 16 are generallyimpermeable to the tissue constituent 22 of interest. The sensor 10A ispermeable to the tissue constituent 22 where the collection chamber 12contacts the tissue 18. When the partial pressure of the tissueconstituent 22 in the sensor 10A is substantially equal to the partialpressure of the tissue constituent in the tissue 18, the sensor 10A isequilibrated. The sensor 10A is arranged to provide circulating flow ofthe tissue constituent 22 through the sensor 10A. Thus, the tissueconstituent 22 may equilibrate throughout the sensor 10A while beingtransferred from the gas collection chamber 12 through the efferentconduit 14 a to contact the sensing component 16, and may return to thegas collection chamber 12 through the afferent conduit 14 b. Such anembodiment may be advantageous when a tissue constituent 22 is beingcontinuously or regularly monitored. As the initial application of thegas collection chamber 12 to the mucosal tissue 26 may involve waitingfor 5-10 minutes before the tissue constituent 22 equilibrates in thegas collection chamber prior to being analyzed, it is desirable to keepthe sensor in the equilibrated state. In such an embodiment, the vent 38to the outside may be closed during equilibration and tissue constituent22 monitoring. The vent 38 may be opened when necessary in order toflush out the sensor with room air or purge gas.

The equilibration time of the sensor 10A may be influenced by certainfactors. Generally, equilibration times may be in the range ofsubstantially instantaneous, i.e., real time, to less than 5 minutes. Incertain embodiments, the response time is in the range of 5 seconds to30 minutes. For example, in certain embodiments, the thickness of abarrier layer 28 may be modified in order to achieve the desired rate ofcarbon dioxide perfusion and sensing component 16 response time. Where avery rapid response is desired, a thin film of the barrier layer 28, forexample less than 0.2 mm in thickness, may be used. Additionally, thebarrier layer 28 may be formed with small pores that increase the carbondioxide permeability. In other embodiments, the response time may beinfluenced by the volume of the gas collection chamber 12 or the lengthand diameter of the conduit 14. It is envisioned that the volume of thegas collection chamber 12 may be optimized to be large enough to allowsufficient tissue constituents 22 to be collected to obtain accuratemeasurements while being small enough to provide rapid response times.For example, in certain embodiments, the total volume of the gascollection chamber may be 0.2-5.0 cubic centimeters. It may beappropriate to use a relatively smaller, e.g., 0.2-0.8 cubiccentimeters, gas collection chamber on a neonate. In certainembodiments, the total volume of the sensor 10A, including the conduit14, sensing component 16, and pumping system 33 may be 2-500 cubiccentimeters. Generally, smaller sensor 10A volumes are associated withfaster equilibration times.

Referring to FIG. 4, a flow chart 44 illustrates how a tissueconstituent 22 may be analyzed by the sensor 10A. A gas collectionchamber 12 is applied to a patient's mucosal tissue (block 46) and thetissue constituent 22 diffuses into the gas collection chamber 12. Thepump 34 and/or flow regulator 32 is then activated, either by thehealthcare provider or by a processor 40 (block 49 and the tissueconstituent 22 equilibrates while being pumped through the sensor 10A.The tissue constituent 22 is drawn into proximity with sensing component16, and the sensing component 16 provides a signal related to the tissueconstituent 22 (block 50). The tissue constituent 22 may be circulatedthrough the sensor (block 54) back to the gas collection chamber 12 inorder to maintain the equilibrated state.

In certain embodiments, it may be advantageous to provide a sensor witha gas collection portion with a large surface area that contacts thetissue. Such a sensor may equilibrate more rapidly, as tissueconstituent 22 may diffuse more rapidly into the gas collection portion.FIGS. 5A-5B illustrate sensors 10B with alternative gas collectionconfigurations. In FIG. 5A, a sensor 10B may include a gas collectionportion in the form of a coiled tube 80, which may be coiled in themanner of a garden hose, that is permeable to the tissue constituent 22.The coiled tube 80 may increase the available surface area of the gascollection portion of the sensor 10B, as it may be adapted to lay flatagainst a tissue 18. The tissue constituent 22 diffuses into the coiledtube 80 and is drawn into the efferent conduit 14 a. The tissueconstituent 22 may be circulated through conduit 14 b. Although thecoiled tube 80 is permeable to the tissue constituent 22, the tissueconstituent is able to equilibrate in the sensor as the coiled tube 80may adapted to be substantially surrounded by mucosal tissue 18. Forexample, the coiled tube 80 may be placed sublingually. In an alternateembodiment, the coiled tube 80 may be adapted to be permeable only onone side by applying a tissue constituent impermeable coating (notshown) to certain portions of the coiled tube 80. Thus, once the tissueconstituent 22 diffuses into the coiled tube 80, the partial pressure ofthe tissue constituent 22 in the sensor 10B may equilibrate with thetissue 18 without leaking out the portion of the coiled tube 80 not incontact with the tissue 18. Such a configuration may be appropriate foruse on buccal tissue. In an alternate embodiment, shown in FIG. 5B, thetissue constituent permeable collection portion may assume a zigzagconfiguration 82 connected to the conduit 14. Exposed portion of thesubstrate 84, i.e., portions not in contact with the tissue 18, may becoated with a tissue constituent impermeable coating 86 to preventleaking. It should be understood that the configurations shown aremerely exemplary, and the gas collection portions of the sensor 10B maytake any suitable shape, such as a helix, a coiled coil, or otherconfigurations. Appropriate permeable materials from which the permeablegas collection portions may be formed may include Silastic® siliconerubber, available from Dow Coming (Midland, Mich.).

FIG. 6 illustrates a tissue constituent permeable coiled tube 80connected to a pumping and sensing component system 33 by efferentconduit 14 a and afferent conduit 14 b. Efferent conduit 14 a is adaptedto draw the tissue constituent 22 to the sensing component 16. Thesystem may include a calibration element 81 as described herein that isadapted to communicate with the processor 40 and provide informationrelated to the characteristics of the disposable portion of the sensor10D, which may include the tissue constituent permeable coiled tube 80and certain segments of the conduit 14. It is envisioned that anysuitable tissue constituent permeable assembly as described herein maybe connected to the pumping and sensing component system 33 as shown.

The sensors as provided herein may prevent water infiltration into asensing component by arranging a sensor such that the sensing componentis removed from the tissue and thus is removed from bodily fluids.However, in certain embodiments it may be advantageous to provide aunitary sensor configuration including a gas collection chamber on whichor within which the sensing component is disposed. Such an arrangementmay be easier to for a healthcare worker to apply and operate, as itdoes not involve a motive device. Additionally, such a sensor may besmaller and lighter, providing certain transportation and storageadvantages. In such an embodiment, water infiltration into the sensormay be reduced by providing a sensor that includes a water barrierlayer. FIG. 7-FIG. 8 illustrate an alternate embodiment of a tissueconstituent sensor 10D in which the sensor body 55 includes a sensingcomponent disposed proximate to a gas collection chamber 57. FIG. 7shows the sensor applied to a patient. FIG. 8 shows a cross-sectionalview of the sensor 10D. As depicted, water is prevented frominfiltrating the sensor 10D by a barrier layer 58 as described hereinthat forms at least part of a surface of the sensor 10D that contactsthe tissue. In an alternate embodiment (not shown), the sensor 10D maybe configured to prevent water infiltration by a structure that absorbsand/or redirects water away from the sensing components. For example,the sensor 10B may include a water vapor permeable backflush tube thatis selectively permeable to water vapor to allow water vapor to beabsorbed and evaporate away from the sensing components withoutinfiltrating the sensor. Such a tube may include a material such asNafion (available from DuPont, Wilmington, Del.). The barrier layer 58is connected to a housing 56 that, when applied to the mucosal tissue28, forms a collection chamber that traps a tissue constituent 22 thatdiffuses through the barrier layer 58. It should be understood that thesensor 10D may include any sensing component as described herein. Forexample, sensing component may be an optical transducer. In such anembodiment, the trapped tissue constituent 22 may be irradiated by anemitter 60, and the emitted light that passes through the tissueconstituent may be detected by a detector 62. The emitter 60 and thedetector 62 are electrically coupled to a cable 64 by wires 68. Thewavelength of the light emitted by the emitter 60 and the detectionrange of the detector 62 may be selected to detect a wide range oftissue constituents 22. For example, the emitter 60 may also include afilter, for example a 4.26 micron wavelength filter. Such a filter maybe appropriate for use in an embodiment where carbon dioxide ismeasured.

In some embodiments, the sensor 10D is arranged to operate intransmission mode, and casings for the emitter and detector may beformed in the housing 56 on opposite sides of the sensor 10D. In analternate embodiment, the emitter 60 and the detector 62 may be arrangedto operate in reflectance mode (not shown), and can be located on thesame side of sensor 10D. In such an embodiment (not shown), a mirror maybe placed on the opposite side of the housing 56 to reflect theradiation emitted from the emitter 60 back to the detector 62. Whenemploying optical sensing components 16, it may be advantageous todispose an opaque or reflective layer on the tissue-contacting surfaceof the sensor 10D to prevent signal artifacts as a result of theabsorption of a portion of the emitted light by the tissue 18.

In certain embodiments (not shown), the gas collection chamber 57 mayinclude a calibration element 66 or other transducer that may provide asignal related to the volume and other characteristics of the gascollection chamber 57. Such a calibration element 66 may allow adownstream processor or monitor to determine a suitable amount of timeto allow the sensor 10B to equilibrate (i.e. to allow the tissueconstituent 22 to diffuse into the gas collection chamber 57) beforeobtaining accurate measurements related to the tissue constituent 22.Additionally, the calibration element 66 may be a coded resistor orEEPROM or any other suitable device as described herein that providesinformation related to the calibration of any optical sensingcomponents. Such a calibration element 66 may be advantageous inincreasing manufacturing yield of the sensor 10D. For example, a sensor10D including such a calibration element 66 that provides informationabout the emission wavelength or wavelength range of the emitter 60 maybe able to be more accurately calibrated for a wider range of potentialemission wavelengths than a sensor lacking such a calibration element66.

It may be advantageous to provide a sensor 10E as a dipstick-like devicewith a holder 88 that has a familiar and comfortable shape that is easyto use. For example, water-resistant sensors as provided herein may beused in vivo by a patient much like an oral thermometer. FIG. 9illustrates a cross-sectional view of a sensor assembly 10E according tothe present techniques. Such a sensor 10E may be adapted to assess oneor more tissue constituents 22, as illustrated. A barrier layer 90, asdescribed herein, may reduce water infiltration into the sensor 10E. Asillustrated, the sensor 10E includes multiple gas collection chambers,each of which may include a different sensing component 16. For example,sensing components 16 a, 16 b, 16 c, and 16 d may be adapted to eachsense a different tissue constituent 22. In certain embodiments, thedifferent tissue constituents may be carbon dioxide, carbon monoxide,oxygen, and other diffusible gases or volatile compounds. Each of thesensing components 16 may be electrically coupled to a display 94 bywires 92. The display may then indicate the concentrations of the tissueconstituents 22 as measured by the sensing components 16. In analternate embodiment, the sensor 10E may include electrical input andoutput wires (not shown) that may extend along the holder 88 to coupleto a cable, which may be connected to a patient monitor. In anotheralternate embodiment (not shown), such a sensor 10E may be adapted toinclude distal sensing components 16 as described herein and a motiveforce structure to draw the tissue constituent 22 into the distalsensing components. Further, in another alternate embodiment (notshown), the barrier 90 may include a series of selectively permeablebarriers specific for a variety of tissue constituents 22. Thus, each ofthe gas collection chambers 12 may only be permeable to a particulartissue constituent.

The sensor 10E may be inserted into the oral passage and placed adjacentto a mucosal tissue 18. The sensor 10E may be suitably sized and shapedsuch that a patient may easily close his or her mouth around the holder88 with minimal discomfort. In certain embodiments, the sensor 10E maybe adapted to be held against the cheek or any other mucosal tissue. Theholder 88 may also include a handle portion that is accessible fromoutside the mouth and may be manipulated by the patient or a healthcareworker in order to properly position the sensor assembly 10E within themouth.

Sensors as described herein may include any appropriate sensingcomponent for assessing a tissue constituent, including chemical,electrical, optical, non-optical, quantum-restricted, electrochemical,enzymatic, spectrophotometric, fluorescent, or chemiluminescentindicators or transducers. In certain embodiments, the sensing componentmay include optical components, e.g. an emitter and detector pair thatmay be of any suitable type. For example, the emitter may be one or morelight emitting diodes adapted to transmit one or more wavelengths oflight in the red to infrared range, and the detector may one or morephotodetectors selected to receive light in the range or ranges emittedfrom the emitter. Alternatively, an emitter may also be a laser diode ora vertical cavity surface emitting laser (VCSEL). An emitter anddetector may also include optical fiber sensing components. An emittermay include a broadband or “white light” source, in which case thedetector could include any of a variety of elements for selectingspecific wavelengths, for example reflective or refractive elements orinterferometers. These kinds of emitters and/or detectors wouldtypically be coupled to the rigid or rigidified sensor via fiber optics.Alternatively, a sensor may sense light detected from the tissue is at adifferent wavelength from the light emitted into the tissue. Suchsensors may be adapted to sense fluorescence, phosphorescence, Ramanscattering, Rayleigh scattering and multi-photon events or photoacousticeffects. It should be understood that, as used herein, the term “light”may refer to one or more of ultrasound, radio, microwave, millimeterwave, infrared, visible, ultraviolet, gamma ray or X-ray electromagneticradiation, and may also include any wavelength within the radio,microwave, infrared, visible, ultraviolet, or X-ray spectra.

Alternatively, the sensing component may include an active ingredient ofthe indicating element, for example the active ingredient involved inproviding the required response signal when exposed to a givenconcentration of carbon dioxide or other constituents. The activeingredient may be any indicator that is sensitive to the presence ofcarbon dioxide and that is capable of being calibrated to give aresponse signal corresponding to a given predetermined concentration ofcarbon dioxide. The signal may be visual, e.g. a change in color, orelectrical. Indicators which provide a color change in a presence ofcarbon dioxide may include chromogenic pH-sensitive indicators andoxidation/reduction indicators.

A chromogenic pH-sensitive indicator may provide a color change uponexposure to a given concentration of carbon dioxide or other metabolitesin the presence of other ingredients of the element that provide theappropriate chemical conditions to induce the required color change. Forsuch an indicator to be capable of giving a determination of carbondioxide, it is typically used in combination with a suitable base thatprovides an alkaline solution. The hydroxyl ions or amine residuespresent in the alkaline solution react chemically with carbon dioxide toproduce a carbonate, bicarbonate and/or carbamate moiety. The resultingreaction depletes the hydroxyl ion or amine at the interface and thuslowers the pH at the surface of the component impregnated with theindicating element. The lowering of the pH causes a color change in theindicator.

Chromogenic pH-sensitive indicators according to the present techniquesmay include metacresol purple, thymol blue, cresol red, phenol red,xylenol blue, a 3:1 mixture of cresol red and thymol blue, bromthymolblue, neutral red, phenolphthalein, rosolic acid,alpha-naphtholphthalein and orange I. Examples of other indicators whichmay be used include bromcresol purple, bromphenol red, p-nitrophenol,m-nitrophenol, curcumin, quinoline blue, thymolphthalein and mixturesthereof. Suitable bases include sodium carbonate, lithium hydroxide,sodium hydroxide, potassium hydroxide, potassium carbonate, sodiumbarbitol, tribasic sodium phosphate, dibasic sodium phosphate, potassiumacetate, monoethanolamine, diethanolamine and piperidine.

The sensing component may include a semi-conductive sensing element,such as an ion-sensitive field-effect transistor (ISFET). An ISFET mayinclude a silicon dioxide gate for a pH selective membrane. Such asensor may be adapted to sense downstream changes in hydrogen ionconcentration in response to changes in carbon dioxide or other tissueconstituent concentrations.

The sensing component may also include an enzyme-based detection system.For example, one such enzyme may be carbonic anhydrase, which is anenzyme that assists interconversion of carbon dioxide and water intocarbonic acid, protons, and bicarbonate ions. As described above, thisreaction lowers the pH at the surface of the component impregnated withthe indicating element. The lowering of the pH may cause a color changein the indicator. Another such enzyme-based detection system is anenzyme linked immunosorbent assay (ELISA). For example, such an assaymay be appropriate when assessing tissue proteins. Thus, the indicatorelement may include a primary antibody specific for the tissue proteinof interest, and a labeled secondary binding ligand or antibody, or asecondary binding ligand or antibody in conjunction with a labeledtertiary antibody or third binding ligand. The label may be an enzymethat will generate color development upon incubating with an appropriatechromogenic substrate. Suitable enzymes include urease, glucose oxidase,alkaline phosphatase or hydrogen peroxidase.

A chemical indicator may be used in conjunction with an electrical orelectronic device that is adapted to detect and measure changes in theambient chemical parameters induced by the presence of critical amountsof carbon dioxide. For example, optical fiber carbon dioxide sensors maybe used to convert a change in a chemical indicator to a quantitativemeasurement of carbon dioxide in the sample. Generally, such sensorsoperate by directing light of a predetermined wavelength from anexternal source through the optical fiber to impinge the chemicalindicator. The intensity of the emitted fluorescent light returningalong the fiber is directly related to the concentration of carbondioxide in the sample, as a result of the pH-sensitive indicatormaterial present at the fiber tip (i.e., the pH of the indicatorsolution is directly related to carbon dioxide concentration, as aresult of carbonic acid formation). The emitted light is carried by theoptical fiber to a device where it is detected and convertedelectronically to a carbon dioxide concentration value. The sensor mayadditionally have a reference dye present in the indicator composition.The intensity of the light emitted form the reference dye may be used tocompensate, via rationing, the signal obtained from the indicator. Othercomponents may be incorporated into the indicator composition includingsurfactants, antioxidants and ultraviolet stabilizers may also bepresent in the indicator composition. The sensing component may beformed from any appropriate substrate. For example, the sensingcomponent may be filter paper, which may be soaked in, dipped in, orotherwise exposed to the appropriate carbon dioxide-sensing compounds.In certain embodiments, the filter paper may be dipped into a solutioncontaining the indicating compounds on only one side. The sensingcomponent may also be polysulfone, polyproplylene, or other polymersubstrates. The sensing component may be a thin film, or a thickersubstrate. A thicker substrate may lead to a slower response time, whichmay be advantageous in situations in which a sensor is monitoring carbondioxide levels over a longer period of time. Additionally, the sensingcomponent may have pores of a variety of sizes.

The sensing component may include an electrochemical transducer, whichmay be adapted to detect and measure changes in ambient chemicalparameters induced by the presence of critical amounts of a tissueconstituent. In one embodiment, the sensing component may include asensor that employs cyclic voltammetry for carbon dioxide detection.Such sensors are available from Giner, Inc., Newton, Mass. For example,the sensing component may be a thick film catalyst sensor utilizing aproton exchange membrane. Such a sensing component may include thickfilm screen printed electrodes and an electrochemically reversible metaloxide catalysts. Appropriate catalysts include MO, M₂O₃, MO₂, where M isa metal that is any suitable metal, including platinum ruthenium oriridium. Generally, such sensors operate by sensing chemical reactionscaused by proton dissociation from water in which carbon dioxide isdissolved. Dissociated water protons may electrochemically reduce ametal oxide layer of the sensor. The electrochemical reduction of themetal oxide will result in generation of an electrical current, whichvaries in response to the degree of electrochemical reduction.

In another embodiment, the sensing component may include an artificialnose assembly. In such an embodiment, the tissue constituents maycontact an array of electrodes coated with polymers that havecharacteristic electrical properties. The polymers change electricalresistance when contacted with specific volatile materials.

In another embodiment, the sensing component may includequantum-restricted components, including carbon nanotubes, buckeyballs,or quantum dots. Generally, quantum-restricted components may be coatedor otherwise modified with a compound that is sensitive to the tissueconstituent of interest. Interaction of the tissue constituent with thecompound may affect the electrical properties of the quantum-restrictedcomponents such that an electrical feedback may result. In one suchexample, carbon nanotubes may be coated with a carbon dioxide-sensitivecompound or polymer, such as a polyethyleneimine and starch polymer.Carbon dioxide may combine with primary and tertiary amines in thepolyethyleneimine and starch polymer coating to form carbamates. Thechemical reaction lowers the pH of the polymer coating, altering chargetransfer to the carbon nanotubes and resulting in an electrical signalproportional to the pH change. Other suitable polymer coatings may beadapted to sense other tissue constituents of interest, such as oxygenor carbon monoxide. In other embodiments, the quantum-restrictedcomponent may include a binding molecule, such as a receptor or anenzyme that is specific for the tissue constituent of interest. One suchmolecule may include carbonic anhydrase. Binding of the tissueconstituent to its receptor may affect a downstream response that mayresult in a change in the electrical properties of a quantum-restrictedcomponent.

The exemplary sensors, described here generically as a sensor 10, may becoupled to a monitor 70 that may display the concentration of tissueconstituents as shown in FIG. 8. It should be appreciated that the cable72 of the sensor 10 may be coupled to the monitor 70 or it may becoupled to a transmission device (not shown) to facilitate wirelesstransmission between the sensor 10 and the monitor 70. The monitor 70may be any suitable monitor 70, such as those available from NellcorPuritan Bennett, Inc. Furthermore, to upgrade conventional tissueconstituent detection provided by the monitor 70 to provide additionalfunctions, the monitor 70 may be coupled to a multi-parameter patientmonitor 74 via a cable 74 connected to a sensor input port or via acable 76 connected to a digital communication port.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Indeed, the presenttechniques may not only be applied to measurements of carbon dioxide,but these techniques may also be utilized for the measurement and/oranalysis of other tissue and/or blood constituents. Rather, theinvention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by thefollowing appended claims. It will be appreciated by those working inthe art that sensors fabricated using the presently disclosed andclaimed techniques may be used in a wide variety of contexts. That is,while the invention has primarily been described in conjunction with themeasurement of carbon dioxide concentration in blood, the sensorsfabricated using the present method may be used to evaluate any numberof sample types in a variety of industries, including fermentationtechnology, cell culture, and other biotechnology applications.

1. A system comprising: at least one gas collection chamber into which atissue constituent is able to diffuse, wherein the gas collectionchamber is adapted to be placed proximate to a tissue; an efferentconduit adapted to transfer the tissue constituent from the gascollection chamber to at least one sensing component, wherein thesensing component is adapted to provide a signal related to the tissueconstituent; an afferent conduit adapted to transfer the tissueconstituent from the sensing component to the gas collection chamber;and a motive force structure adapted circulate the tissue constituentthrough the system, wherein the motive force structure is adapted to beoperatively connected to at least one of the efferent conduit, theafferent conduit, or the sensing component.
 2. The system, as set forthin claim 1, wherein the tissue constituent comprises carbon dioxide,oxygen, ethanol, or carbon monoxide.
 3. The system, as set forth inclaim 1, wherein the tissue constituent comprises a volatile anestheticagent, a volatile product of metabolism, or a volatile xenobiotic. 4.The system, as set forth in claim 1, comprising a calibration elementadapted to provide at least one signal related to at least one physicalcharacteristic of the gas collection chamber.
 5. The system, as setforth in claim 4, wherein the calibration element comprises a codedresistor or an electrically erasable programmable read-only memory. 6.The system, as set forth in claim 1, comprising a barrier layer definingat least part of a surface of the gas collection structure, wherein thebarrier layer is substantially impermeable to water.
 7. The system, asset forth in claim 1, comprising a barrier layer defining at least partof a surface of the gas collection structure, wherein the barrier layeris selectively permeable to the tissue constituent.
 8. The system, asset forth in claim 1, comprising a water-permeable conduit that isimpermeable to the tissue constituent, wherein the water-permeableconduit is adapted to direct water away from gas collection structure.9. The system, as set forth in claim 1, wherein the sensing componentcomprises a non-optical transducer, an optical transducer, a chemicalindicator, a spectroscopic transducer, or an electrochemical transducer.10. The system, as set forth in claim 1, wherein the gas collectionstructure comprises a tube, wherein at least a portion of the tubeadapted to be placed proximate to the tissue is permeable to the tissueconstituent.
 11. The system, as set forth in claim 1, wherein thesensing component comprises a calibration element adapted to provide asignal related to the calibration characteristics of the sensingcomponent.
 12. The system, as set forth in claim 1, wherein the motiveforce structure comprises a pump, a one-way valve, a kinetic motionstructure, or a piezoelectrically powered structure.
 13. The system, asset forth in claim 1, comprising an agent adapted to be placed proximateto the tissue, wherein the agent is adapted to increase blood flow tothe tissue or wherein the agent is adapted to increase the tissue'spermeability to the tissue constituent.
 14. The system, as set forth inclaim 13, wherein the agent comprises an electrical heating element, achemical heating element, nicotinic acid, or salicylic acid.
 15. Amonitoring device comprising: a monitor; and a system adapted to becoupled to the monitor, the system comprising: at least one gascollection structure adapted to be placed proximate to a tissue; and anefferent conduit adapted to transfer gas from the gas collectionstructure to a sensing component, wherein the sensing component isadapted to provide a signal related to a tissue constituent; and anafferent conduit adapted to transfer gas from the sensing component tothe gas collection structure.
 16. The monitoring device, as set forth inclaim 15, wherein the tissue constituent comprises carbon dioxide,oxygen, ethanol, or carbon monoxide.
 17. The monitoring device, as setforth in claim 15, wherein the tissue constituent comprises a volatileanesthetic agent, a volatile product of metabolism, or a volatilexenobiotic.
 18. The monitoring device, as set forth in claim 15,comprising a calibration element adapted to provide at least one signalrelated to at least one physical characteristic of the gas collectionchamber.
 19. The monitoring device, as set forth in claim 18, whereinthe calibration element comprises a coded resistor or an electricallyerasable programmable read-only memory.
 20. The monitoring device, asset forth in claim 15, comprising a barrier layer defining at least partof a surface of the gas collection structure, wherein the barrier layeris substantially impermeable to water.
 21. The monitoring device, as setforth in claim 15, comprising a barrier layer defining at least part ofa surface of the gas collection structure, wherein the barrier layer isselectively permeable to the tissue constituent.
 22. The monitoringdevice, as set forth in claim 15, comprising a water-permeable conduitthat is impermeable to the tissue constituent, wherein thewater-permeable conduit is adapted to direct water away from gascollection structure.
 23. The monitoring device, as set forth in claim15, wherein the sensing component comprises a non-optical transducer, anoptical transducer, a chemical indicator, a spectroscopic transducer, oran electrochemical transducer.
 24. The monitoring device, as set forthin claim 15, wherein the gas collection structure comprises a tube,wherein at least a portion of the tube adapted to be placed proximate tothe tissue is permeable to the tissue constituent.
 25. The monitoringdevice, as set forth in claim 15, wherein the sensing componentcomprises a calibration element adapted to provide a signal related tothe calibration characteristics of the sensing component.
 26. Themonitoring device, as set forth in claim 15, wherein the motive forcestructure comprises a pump, a one-way valve, a kinetic motion structure,or a piezoelectrically powered structure.
 27. The monitoring device, asset forth in claim 15, comprising an agent adapted to be placedproximate to the tissue, wherein the agent is adapted to increase bloodflow to the tissue or wherein the agent is adapted to increase thetissue's permeability to the tissue constituent.
 28. The monitoringdevice, as set forth in claim 27, wherein the agent comprises anelectrical heating element, a chemical heating element, nicotinic acid,or salicylic acid.
 29. The monitoring device, as set forth in claim 15,comprising a multi-parameter monitor.
 30. A method comprising:transferring a tissue constituent in a gas collection chamber to atleast one sensing component not located in the gas collection chamber,wherein the sensing component is adapted to provide a signal related tothe tissue constituent.
 31. The method, as set forth in claim 30,comprising contacting the tissue constituent with a barrier layerdefining at least part of a surface of the gas collection structure,wherein the barrier layer is substantially impermeable to water.